Looking for something else, I came across a web page that makes this rather startling claim:

[W]alking actually uses more fossil energy than driving, if the calories burned from walking come from a typical American diet.

The crux of the claim is that the North American food system uses so much fossil fuel — for manufacturing fertilizer and pesticides, running farm machinery, transporting food from farm fields to stores and homes, powering refrigerators and stoves, etc. — that producing the food calories to power a one mile walk uses up more fossil fuel energy than a typical car burns in a one-mile drive.

That seemed counterintuitive, to be sure — but not completely ridiculous. So I spent some time looking at the issues.

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As far as I can tell, the web page is probably wrong: walking is more energy-efficient than driving.

However, they’re closer than I might have thought.

Some quick calculations: according to the Earth Policy Institute, the U.S. food system consumed about 10.25 quadrillion BTUs in fossil fuels in 2002 — the energy equivalent of about 3/4 of a gallon of gas for every American, each and every day. (Very little of that energy is actually gasoline, by the way.) Based on this, the U.S. food system burns about six to seven times as many calories of fossil fuel per day as we consume in food.

In other words, we don’t just eat food; we also eat oil, coal, and natural gas.

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(There’s some uncertainty in the figures, to be sure; and I’m still trying to figure out a wrinkle or two in the data. I’ll post more if I find anything interesting.)

Obviously, your mileage may vary. If you eat lots of grain-fed meat, in particular, you’re likely to consume more energy in your diet. Then again, there’s practically no diet that’s truly benign; there are just darker and lighter shades of gray.

As for walking: for a 150 pound person, walking a mile burns about 43 calories above and beyond what the body would burn just loafing around. (See, e.g., here for calories per minute per pound of body mass for walking; and see here for what you burn while watching the tube.) Doing the math — and accounting for the fact that about three food calories are wasted for every seven that are actually consumed (see, e.g, fig. 10, p. 30 of this big ol’ pdf) — I get that a person who walks a mile gets the equivalent of about 75 mpg.

That is, if you lump together all the fossil fuels that go into growing, transporting, selling, storing, and cooking your food, the human body uses a little less fossil fuel, mile for mile, than a high-tech Honda Insight.

However, walking is arguably less efficient — in per-passenger terms, at least — than an Insight with someone riding shotgun. In fact, in one way of looking at things, walking a mile is about as fuel efficient as driving a 15 mpg SUV with all five seats filled. Go figure.

Still, if you’re weighing whether to walk to the store or drive, walking is clearly the environmental winner — and it’s healthier, to boot. And more to the point, a neighborhood where you can do lots of your chores conveniently on foot is bound to be a fuel-efficient place — not just because you can walk or bike (which is more efficient than driving), but also because, day to day, you don’t have to travel far to get to where you need to go.

And the real efficiency comes from arranging our lives so that we don’t have to travel so far every day. That’s where the rubber (or shoe leather) really meets the road.